1. Field of the Invention
The present invention relates generally to an electroplating system and method for electroplating a deposit of metal onto the internal surface of a closed or open hole or other cavity in a workpiece of conductive or partially conductive material, and is particularly concerned with electroplating holes or cavities of high aspect ratio, i.e. holes which are deep compared to their diameter, such as solder cups of electrical terminals of electrical or hybrid electrical and optical connectors.
2. Related Art
Electroplating is a process that uses electrical current to reduce dissolved metal cations to form a metal coating on an electrode. Conventionally, the process involves immersing the part to be plated and an anode (made of the metal to be plated on the part or a noble metal) in a bath or electrolyte solution containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. The part to be plated acts as the cathode of the circuit. As power is supplied to the circuit, metal atoms dissolve in the solution from the anode and are reduced at the interface between the solution and the cathode, such that they “plate” onto the cathode.
For example, electrical and electronic assemblies of instruments and controls are often supplied with electrical connections in the form of metal contact pins (and sockets). The inboard end (i.e., opposite the mating end) of the pins (and sockets) are typically intended for solder connections to electrical wiring. In particular, the soldering operations include soldering the termination of an electrical wire into a “cup” receptacle of the electrical connector thus forming the electrical couple therebetween.
For high reliability applications, the ability to perform the soldering operations (e.g., connecting conducting lines to connector terminals) must be assured, even after long storage of the assemblies and components. To illustrate, if the pins are composed of nickel, stainless steels or other metals that strongly passivate by forming a tough surface oxide layer, they become difficult to solder unless the surface oxide formation is inhibited. Thus, for high reliability applications, the soldering surfaces of such pins are usually provided in an oxide free state that is protected by the pre-soldering application of a layer of gold plating, deposited by electrochemical or other means. If the gold plating is adequately thick (e.g., exceeding approximately 30 micro-inches) and non-porous, it may prevent oxidation of the soldering surface beneath it until the soldering operation is performed. The gold or other plating metal is typically dissolved into the solder alloy during the solder operation and plays no further electrical or mechanical role in the formation of the joint, as long as the concentration of the gold in the solder is not excessive.
Conventionally, the initial plating operations for the finishing of pins featuring solder cups can be performed in several ways. As described above, pins can be electrodeposited with rack or barrel plating fixtures, wherein the pins or the connector assembly is placed in a bath of plating solution. The parts to be electroplated are connected electrically to an electrical power supply negative terminal (−). The bath also contains anodes, connected to the power supply positive terminal (+). The anodes may be of two types: consumable anodes and noble material anodes. Consumable anodes are eroded by the passage of plating current and dissipate into the solution, replacing the metal ions that are deposited on the workpiece. Noble anodes consist of platinum, titanium or similar metal, possibly with coatings applied, or carbon. They do not erode and do not replace the metal ions lost in the process. In this case, the deposited metal is supplied directly from the solution. Electroless plating may also be used, in which the surfaces to be plated are treated with a series of solutions that alternatively activate the surface and then deposit metal on it.
While electroplating in a plating solution bath is adequate for many workpiece geometries, electroplating a uniform deposit in deep hole is difficult if the holes are deep compared with the diameter, referred to as a high aspect ratio. The current flow required for metal deposit is hampered by the longer travel distance required for it to reach the bottom of the hole. The current seeks the closer surface in the travel through the solution, rather that reach the deeper surfaces of the hole. The metal plating therefore plates more heavily on the surfaces near the entrance of the hole, leaving an insufficient deposit at the bottom. The deposition is further hindered by stagnation of plating solution deep in the hole. Careful agitation and fluid flow is required to replenish the spent solution at the bottom of the hole after the latent metal has been deposited from it. Moreover, these challenges are exacerbated in smaller geometries such as commonly found in electronics, electrical connectors, and the like, and solder cups of electrical connectors and the like are often deep enough to present difficulty with obtaining a good metal deposition to the bottom of the cup. This can result in a protective layer which is too thin or poorly adherent, as a result of the features and geometry of the high aspect ratio, deep hole or solder cup.
Additionally, re-plating of holes or cavities with poor or ineffective coating or solder layers in an electroplating bath is generally not feasible for components already assembled into a finished product, such as solder cups of pins already incorporated in an electrical instrument assembly, because the complete assemblies cannot be subjected to the chemicals and temperatures typical of the electroplating process. Thus, such assemblies cannot be immersed in a plating tank or barrel.